The Pitting Resistance Equivalent Number (PREN) is a theoretical way of comparing the pitting corrosion resistance of different metals from their chemical composition. It is useful for ranking grades against each other, but it cannot predict how a grade will actually perform in a specific application.
The PREN formula
The most widely accepted version uses chromium, molybdenum and nitrogen content:
PREN = %Cr + 3.3 x %Mo + 16 x %N
Because most product specifications allow a range of compositions, some end users set a minimum PREN value to ensure adequate corrosion resistance.
Comparing grades
More highly alloyed grades generally have a higher PREN and therefore better resistance to pitting. For example, the PREN values of 316L, 2205 duplex and super duplex stainless steels are 25, 32 and >40 respectively, increased mainly by higher chromium content.
| Grade | PREN |
|---|---|
| Alloy 316L | 25 |
| 2205 duplex | 32 |
| Super duplex stainless steel | >40 |